1. Field of the Invention
This invention relates to an electrophotographic photoconductor for use in a printer, a copier, or the like of the electrophotographic type. More specifically, the invention relates to an electrophotographic photoconductor and which contains a charge generation material and a charge transport material which assign improved electrophotographic characteristics to the electrophotographic photoconductor. The invention also relates to an electrophotographic device using the electrophotographic photoconductor.
2. Description of the Related Art
An electrophotographic photoconductor (may be referred to hereinafter as a "photoconductor") has a basic structure in which a photosensitive layer having a photoconductive function is laminated on a conductive substrate. In recent years, research and development have been performed energetically of organic electrophotographic photoconductors using organic compounds as functional components engaged in generation and transport of an electric charge. The photoconductors have advantages, such as the diversity of available materials, high productivity, and safety, and their application to copiers or printers is under way.
A photoconductor is required to have the function of retaining a surface charge in the dark, the function of accepting light to generate a charge, and the function of transporting the generated charge. Photoconductors having these functions are classified into a single-layered photoconductor, and a separated-function laminated type photoconductor. Concretely, the single-layered photoconductor allocates all of these functions to a single photosensitive layer. The separated-function laminated type photoconductor, on the other hand, comprises a photosensitive layer which is a laminated structure comprising a charge generation layer mainly responsible for the function of generating a charge upon receiving light, and a charge transport layer with the function of retaining a surface charge in the dark, and the function of transporting the charge generated in the charge generation layer during acceptance of light. The separated-function laminated type photoconductor, in particular, is a separated-function photoconductor comprising the photosensitive layer that is separated to the charge generation layer and the charge transport layer.
Recently, the above-described separated-function laminated type photoconductor using organic compounds has found main use as an electrophotographic photoconductor. The photoconductor has a photosensitive layer formed, for example, in the following manner: An organic pigment as a charge generation material is dissolved or dispersed in an organic solvent together with a resin binder to prepare a coating fluid. The coating fluid is applied as a film to form a charge generation layer. Separately, an organic low molecular compound as a charge transport material is dissolved or dispersed in an organic solvent together with a resin binder to prepare a coating fluid. The coating fluid is applied as a film to form a charge transport layer. These two layers are laminated to form a photosensitive layer.
The current organic photoconductors, however, are not enough to fulfill characteristics required of photoconductors. In particular, equipment of a reversal development system adapted to digitization in recent years is constituted so that primary charging and transfer charging will give opposite polarities. Thus, there occurs the phenomenon that the amount of charging of the photoconductor differs according to the presence or absence of transfer, i.e., a phenomenon called transfer memory. This is an unfavorable phenomenon causing image variations. The equipment of the reversal development system faces the problem that this phenomenon tends to occur. A copier in which transfer voltage is always applied to the photoconductor will be taken as an example for explanation. With such a copier, there are a case in which transfer voltage is applied to the surface of an electrophotographic photoconductor via a fed sheet of paper, and a case in which transfer voltage is applied directly to the surface of an electrophotographic photoconductor in a space between a preceding sheet and a sheet to be fed subsequently (the space is called "the intersheet space"). Thus, the difference in the amount of charging occurs between a photoconductor portion having received transfer voltage via the sheet and a photoconductor portion having directly received transfer voltage in the intersheet space. This difference results in a difference in surface potential during a subsequent charging process, presenting the cause of a change in printing density.
Factors for producing the above-mentioned phenomenon may be as follows: During a transfer process of the photoconductor, the charge transport layer on the surface of the photoconductor is first ionized, and makes hole carriers under the action of an electric field. These hole carriers move from the surface of the charge transport layer into the film under the electric field, and are retained there. The hole carriers in the film move to the surface at the time of a next charging process, canceling out the surface charge, and increasing the printing density.
In response to this problem, a method for improvement has been worked out, such as turning off the transfer voltage in the intersheet space on the machine process side. However, this method involves the problem of leading to a cost increase.